The waveform modulation schemes of the air interfaces of the 2G and 3G mobile communication systems use fixed parameters. For example, the 2G GSM (Global System for Mobile communication) system uses signal-carrier GMSK (Gaussian Minimum Shift Keying) modulation with fixed Gaussian filter. The 3G WCDMA (Wideband Code Modulation Multiple Access) system uses single-carrier QAM (Quadrature Amplitude Modulation) modulation with fixed Root-Raised Cosine (RRC) filter. The 4G LTE (Long Term Evolution) uses CP-OFDM (Cyclic-Prefix Orthogonal Frequency Division Multiple Access) multi-carrier waveform with fixed sub-carrier spacing and two options of the Cyclic Prefix (CP) lengths. Table 1 gives an overview of currently specified waveform modulation schemes.
TABLE 1Comparison of the Pulse ShapingParameters of 2G to 5G waveforms3G2G GSMWCDMA4G LTE5GTX filterFixed,Fixed, RRCFixed,VariousGaussianfilterrectangulardesign prin-windowciple, flex-ible lengthof responseOversamplingFixed, 1.5Fixed, 1.22Fixed, 1.07 orFlexiblerate of TX1.25 (addingfilterCP is actuallyoversampling)SubcarrierFixed,Fixed,Fixed, 15 kHzflexiblespacingsinglesinglecarriercarrier
In the on-going research and design of the future 5G mobile communication system, the air interface with highly flexible waveform configurations are being intensively studied for achieving new features, such as: For supporting the diverse and mixed services for machines and human; for adapting diverse channel and multiple access models; for slicing highly efficiently the radio spectrum resource; for composing reconfigurable frame structure, especially for short-latency application; and for achieving physical layer security against eavesdropping.
For providing the air interface with highly flexible waveform configurations, the chosen configuration can be either signaled between transmitter (TX) and receiver (RX), e.g. by using Common Control Channel Signaling, or the receiving unit can apply blind recognition methods to detect the used waveform configuration. However, signaling requires additional resources for the signaling, e.g. an external signaling channel and blind detection requires long observation time and tends to be unreliable, in particular with fading channel and clock imperfection which makes it inapplicable to short frame and practical conditions.